Low-friction carbon-rich carbide coatings deposited by co-sputtering

Low-friction coatings are used more and more frequently, particularly in situations and applications with insufficient or no lubrication. A good example of such coatings is amorphous carbon, which is produced both in pure form (a-C:H) and doped with metal (Me-C:H). The knowledge of what actually occurs when one metal in a Me-C:H coating is exchanged with another has so far been rather limited. Also, when producing these films hydrogen is incorporated in the substrate as well as in the film, which can be detrimental to the overall properties. Here, a newly adopted co-sputtering technique, utilizing a carbon target partly covered by metal-foil strips, was used to deposit non-hydrogenated carbon coatings alloyed with Ta, W and Zr on ball-bearing steel (BBS) substrates. The metal content varied between 0 and 41 at.%, and the resulting films were analyzed with respect to phase composition and textures, chemical composition, microstructural morphology, as well as mechanical and tribological properties. All alloyed coatings displayed a nanocomposite microstructure, with 3–6 nm metal-carbide crystallites embedded in a matrix of amorphous carbon. The amount of metal-carbide phase increased with increasing amounts of metal which led to a large increase in hardness and elastic modulus. An increased metal content did however not affect the carbide size to any notable extent. Ball-on-disk tests show that metal additions cause a sharp drop in friction coefficient from 0.21 to about 0.05, depending on the metal used. This is however accompanied by an increase in wear rate. The coating best combining low friction and low wear rate was alloyed with 20 at.% Ta. Best possible protection of the counter surface was offered by coatings containing 30 at.% Ta or more.